Patient-specific surgical guides become more and more used in dentistry or orthopedic surgery, for example in view of implanting total knee prosthesis.
A patient-specific guide is generated by an additive manufacturing technique (e.g. stereolithography) by including two kinds of elements:                contact elements intended to match an anatomical structure (e.g. a bone) to be treated; and        guiding elements such as drill guides, saw guides, or milling guides, intended to guide a surgical instrument to carry out the desired treatment once the patient-specific guide is positioned onto the anatomical structure of the patient. The planning of the position of the guiding elements corresponds to the planning of the intended treatment of the anatomical structure.        
The contact elements are chosen so as to provide a unique and stable position of the guide with respect to the anatomical structure.
FIG. 1 is a schematic view of an example of a patient-specific guide 1 positioned onto a patient's anatomical structure 2.
The guide 1 comprises a contact element 11 having a surface in contact with the anatomical structure 2, a guiding element 12 for a saw blade and a guiding element 13 for a drill.
WO 93/25157 describes a method for constructing a patient-specific surgical guide.
A 3D medical image (e.g. CT or MRI) of an anatomical structure of the patient is first segmented so as to reconstruct the anatomical structure, i.e. to form a 3D model of the anatomical structure. Such a 3D model is a representation of the 3D surface of the anatomical structure (for example using triangular facets) or a representation of the volume of the anatomical structure (for example using voxels) which implicitly defines its surface.
Then, contact points and/or contact faces are defined on the surface of the reconstructed anatomical structure so as to provide unique and stable positioning of the guide.
On the other hand, the position of the guiding elements with respect to the anatomical structure is defined.
Then, the surgical guide is constructed by generating a rigid body including the guiding elements and the contact elements. By “rigid” is meant here that the guide is not intended to deform during the surgical intervention.
The surgical guide can then be produced by an additive manufacturing technique.
Such a method is long and expensive for the following reasons.
In practice, it involves several flows of data between a radiologist who has acquired the 3D medical image, an expert center that carries out the segmentation of the 3D medical image and the planning of the surgical guides, and the surgeon who has ordered the patient-specific guide.
Typically, at least four flows of data and/or material are to be considered in such a process:
(A) The 3D medical image is sent by the radiologist to the expert center that carries out a segmentation of the 3D medical image so as to reconstruct the anatomical structure and determines a planning comprising a proposed position of the guiding elements.
The expert center usually comprises experts (engineers and/or technicians) in the processing of medical images.
The experts use specific tools for facilitating the segmentation of the images.
However, since the 3D medical image usually comprises a plurality of slices—typically from 150 to 200 slices—an error in the segmentation of only one slice may generate a large error in the final result.
Hence, the segmentation cannot be completely carried out automatically, and the expert has to segment manually at least the regions of the 3D medical image where the greyscale impedes an automatic recognition of the pixels between bone and soft tissues.
Such a manual segmentation is time-consuming (sometimes several hours) and increases the cost of the surgical guide.
The planning is usually based on standard default parameters.
(B) The expert center sends the planning to the surgeon.
(C) The surgeon checks and, if necessary, modifies the planning.
However, depending on the format of the planning data provided by the expert center, it may be difficult and unpractical for the surgeon to modify the planning.
Hence, the surgeon may be incited to accept the planning as provided by the expert who is usually not a surgeon; this situation is not satisfactory in terms of involvement of the surgeon in the planning step and more specifically in terms of responsibility.
(D) Based on the planning and the segmented image, the expert center constructs the surgical guide.
Said construction typically relies on the subtraction of the volume of a body comprising the guiding elements and intersecting the anatomical structure on the one hand, and of the volume of the anatomical structure.
Then, the expert center manufactures the guide (or orders it to a dedicated manufacturing center) and sends it to the surgeon.
Although such a patient-specific guide strongly reduces the time spent in the operating room and drastically reduces the instrumentation required for carrying out the surgical intervention, many surgeons report that they have some difficulties in accurately positioning the guide onto the anatomical structure.
A slight mismatch between the surface of the contact element of the guide and the respective surface of the anatomical structure may result in an angular error of several degrees in the orientation of the guiding elements.
Such a mismatch may be caused by an inaccurate segmentation of the 3D medical image (in particular in osteophytes) or by soft tissues.